Am Nat. 2022 Jan;199(1):75-90. doi: 10.1086/717175. Epub 2021 Nov 23.
AbstractAll else equal, parasites that harm host fitness should depress densities of their hosts. However, parasites that alter host traits may increase host density via indirect ecological interactions. Here, we show how depression of foraging rate of infected hosts can produce such a hydra effect. Using a foraging assay, we quantified reduced foraging rates of a zooplankton host infected with a virulent fungal parasite. We then parameterized a dynamical model of hosts, parasites, and resources with this foraging function, showing how foraging depression can create a hydra effect. Mathematically, the hydra arose when increased resource productivity exceeded any increase in resource consumption per host. Therefore, the foraging-mediated hydra effect more likely emerged (1) for hosts that strongly control logistic-like resources and (2) during larger epidemics of moderately virulent parasites. We then analyzed epidemics from 13 fungal epidemics in nature. We found evidence for a foraging-mediated hydra effect: large outbreaks depressed foraging rate and correlated with increased densities of both algal resources and hosts. Therefore, depression of the foraging rate of infected hosts can produce higher host densities even during epidemics of parasites that increase host mortality. Such hydras might prevent the collapse of host populations but also could produce higher densities of infected hosts.
摘要
在其他条件相同的情况下，损害宿主适应性的寄生虫应该会降低宿主的密度。然而，改变宿主特征的寄生虫可能会通过间接的生态相互作用增加宿主密度。在这里，我们展示了感染宿主觅食率下降如何产生这种九头蛇效应。使用觅食实验，我们量化了感染了一种烈性真菌寄生虫的浮游动物宿主的觅食率下降。然后，我们用这个觅食功能来参数化一个宿主、寄生虫和资源的动态模型，展示了觅食抑制如何产生九头蛇效应。从数学上讲，当资源生产力的增加超过每个宿主的资源消耗的任何增加时，九头蛇就出现了。因此，觅食介导的九头蛇效应更可能出现（1）对那些强烈控制逻辑资源的宿主，以及（2）在中度致命性寄生虫的较大流行中。然后，我们分析了 13 种自然发生的真菌流行病中的流行病。我们发现了觅食介导的九头蛇效应的证据：大爆发降低了觅食率，并与藻类资源和宿主的密度增加相关。因此，即使在增加宿主死亡率的寄生虫流行期间，感染宿主觅食率的下降也可以产生更高的宿主密度。这样的九头蛇可能会防止宿主种群的崩溃，但也可能会产生更高密度的感染宿主。
